A digital circuit such as a microprocessor has numerous transistors that alternate between dormant and switching states. Such digital circuits thus make abrupt current demands when large numbers of transistors switch states. But power supplies cannot react so quickly such that the voltage on the power supply lead or interconnect to the die including the digital system may dip unacceptably. To smooth the power demands, it is conventional to load the power supply lead with decoupling capacitors. The decoupling capacitors store charge that may be released during times of high power demand so as to stabilize the power supply voltage.
Decoupling capacitors typically mount to the package substrate or to the circuit board and connect to the die through the power supply and ground interconnects or leads. The interconnect distance between the decoupling capacitor and the die introduces parasitic inductance, which undesirably increases the impedance. In addition, the decoupling capacitors demand valuable package substrate or circuit board space. Integrating the decoupling capacitors into the die itself is also undesirable because the decoupling capacitors will then lower density and increase costs. Alternatively, decoupling capacitors have been formed using film capacitors on an interposer. The die mounts to the interposer, which lessens the parasitic inductance because the decoupling capacitor is then closer to the die. But the capacitance per unit area is too small to make such thin film capacitors an effective alternative. The use of embedded passive capacitors within the package substrate represents another alternative. But the die couples to such embedded passive capacitors with just a pair of vias, which increases the parasitic inductance despite the relative closeness of the embedded passive capacitors. Moreover, the formation of the cavity and embedding of the capacitor into the cavity is relatively expensive and cumbersome.
Accordingly, there is a need in the art improved capacitor designs for power distribution and other applications.